Automatic fidelity control circuits



2 Sheets-Sheet l Sept. 22, 1936.

NKJV MLWWMIS N s Nw NTMWHIMMIWW d@ N 'BV/QZ /uflfm/ ATTORNEY sept. 22, 1936. vR A BRADEN 2,054,892

AUTOMATIC FIDELITY CONTROL CIRCITS S14/At?? F AMP INVENTOR RENE A.BRADEN ATTORN EY Patented Sept. 22, 1936 UNITED STATES 2,054,892 AUTOMATIC FIDELITY CONTROL CIRCUITS Rene A. Braden, Collingswood, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application November 29, 193s, serial No. 700,247

9 Claims.

My present invention relates to fidelity control arrangements for radio frequency signalling systems, and more especially to automatic fidelity control circuits for radio receivers.

In by co-pending application Serial No. 682,743, led July 29, 1933 there' are disclosed, and claimed, various arrangements for automatically regulating the effective resonance curve of a tunable network in response to the variation in amplitude of received signal energy. The essential aim of these arrangements is to control the selectivity of a radio frequency signalling system in such a manner that the system is more selective on weak signals than on strong ones with the result that on strong local stations the selectivity is least, but on weak stations the receiver is most selective.

` Now, in the present application there are disclosed further arrangements for automatically varying selectivity and improving fidelity when receiving strong signals. In each of the Ypresent arrangements a double system of amplifiers is utilized, certain of the amplifiers being operative, in response to variations in amplitude of the received signal energy, to improve. the selectivity of the receiver and impair its fidelity when receiving distant station signals, and other of the amplifiers being operative to decrease the selectivity and improve the fidelity of the receiver when receiving strong signals as from local stations.

Hence, it may be stated that it is one of the main robjects of the-present invention to provide further automatic fidelity co-ntrol arrangements, the arrangements functioning to vary the fidelity of a radio receiver by utilizing, in responseto variations in amplitude of received signal energy, amplifiers of predetermined selective characteristics.

Another important object of the present invention is to provide at least two parallel amplifiers in a radioreceiver, one of which amplifiers has a sharp selective characteristic and the other has a broad characteristic, the receiver additionally including means for differentially controlling the transmission efficiency of said amplifiers in such a manner that the amplifier of sharp selectivity is dominant when receiving weak signals, and the amplifier of broad selectivity predominates in action when receiving stro-ng signals.

Another object of the invention is to provide in a radio receiver a push-pull selective amplifier which has a sharp resonance curve in normal operation, but which includes means for automatically rendering certain of the tubes of the push-A Vwherein like reference characters in the different pull amplifier Vinoperative for broadening the I resonance curve when receiving strong signals, as from local stations. Y

Still other objects of the invention are to improve generally automatic delity control cirf cuits for radio receivers, and to provide such control circuits which are not only reliable in operation, but readily constructed and assembled in receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation .will best be understood by an embodiment figures designate similar circuit elements, there is shown in Fig. 1, in purely conventional form, a radio receiving system embodying one form of the invention. The receiving system comprises a source of signal energy I, which source may be the signal collecting means and radio frequency amplifier o f .a superheterodyne receiver. The sourceof signals is shown followed by a network 2 which may comprise a combined oscillatordetector circuit of any type well known to those skilled in the art, or a first detector upon which is impressed local oscillation energy from any well known type of local oscillator. In any case the circuit I in the output of the network 2 is tuned to the operating intermediate frequency and is coupled, as at M1, to the tunable input circuit II of the intermediate. frequency amplifier 3, preferably of the screen grid type, the circuit II being tuned to the operating intermediate frequency. y

The tube 3 includes in its anode circuit a resonant circuit III having one side thereof connected to a source of positive potential B (not shown), this tuned circuit being tuned to the operating intermediate frequency and being coupled, as at M2, to a circuit IV which is tuned to the operating intermediate frequency. The circuit IV is connected to the input electrodes of the second detector 4, and this tuned circuit includes a coil 5 in series with the tuning condenser and secondary winding of the coupling transformer M2.

The coupling M1 is less than critical so that the resonance curve at `the grid of tube 3 is relatively sharp, and the sameapplies to M2. Signal energy of intermediate frequency is also fed to a screen grid tube 6 from the tuned circuit I through av path which includes the condenser 7. The cathode of tube 6 is connected to the low potential` side of circuit I through ground, while the condenser 'I connects the highpotential side of circuit I to the control grid of tube 6.

The anode circuit of tube 6 includes a resonant circuit V coupled, as at M3, to a resonant circuit! VI, the latter circuit being connected between `the input electrodes of the screen grid amplifier tube II, and the anode circuit of the tube I I including a resonantcircuit-VII; The coil 5 of the rectifier input circuit IV is magnetically coupled to the circuit VII.r Circuits V, VI and VII are tuned to the operating intermediate frequency.

The-direct current blocking condenser I5 provides a coupling path to the Vsucceeding audio amplifierfand/ or reproducer for the audio component of the detected intermediate frequency energy. 'I'he lead I6 fconnects the grid circuit of tube 3 to a source of -gain control potential designated AVC, and itis to be clearly understood that the source may be any rectier arrangement well knownV to those skilled in the art, and commonly employed for automatic volume control.

The resonance curve at the grid of tube 6 has a substantially fla-t top, and is broader than the resonance curve at the grid of tube 3. The control biases impressed upon tubes 6 'and 3 through leads II and I6 respectively are arranged to work differentially. The llead IIis connected between the grid of tube 6 and another source of control potential. With an extremely strong signal the amplification through `tube 3 is completely cut-off, and the signals come' through tubes I3` and I I. The outputs of tubes 3 and II are fed into the com-- mon input circuit IV of the detectorv 4, although these outputs of tubes 3 and I I may be separately rectified and-combined in the audio circuit.

The differential biasing of the sharp and broad amplifiers is such that the gain of the sharpampliiier isgreater than that-of the broad amplifier when weak signals are received; while the reverse is true when strong signals are received. In between-these two conditions there vwill be a value ofisignal strength at which the amplifiers have equal gains, and an intermediate degree of selectivity'is obtained. The control means I6 feeding the grid-of tube 3 may be a biased detector for example, and the plate direct current potential component of the latter would then be used to reduce Ythe ga-in of -the `sharp amplifier with high signal input. The control device Il' can be a-grid lead detector, in which case athigh signal input the broad amplifier would operate at maximum gain because the plate direct current potential component is a minimum at such high input. With weak signal input the sharp amplifier operates with maximum gain, and the broad amplifier has a minimum gain. Any other desired type of gain control devices could be used at I6 and Il" provided the action was similar.

It will be seen that there has been provided between the detector 4-and the first detector 2 Vtwo parallel amplifier systems, both operating at intermediate frequency. One of these systems is of sharp selectivity, while the other system is of broad selectivity. The signal from the first detector can be made to go through either of these Yparallel amplifier systems7 or to divide in any proportion and go through both amplifiers. The connections from the two control sources I5-i'! to the two `parallel amplifier systems function to differentially operate these parallel ampliiier systems, and the diiierential biasing is accomplished lin dependence on the signal amplitude variations. This has the result that the selectivity and fidelity ofthe receiver varies in accordance with the received vsignal strength.

To increase the Variation of selectivity with signal strength, one or more additional stages of amplication may be inserted between the tube 3 and the detector 4', and between the tube 6 and therectifier 4. Where such additional stages are utilized the couplings in the system including tube 3 should be less than critical, while the couplings in the system including tube 6'should be greater than critical coupling.

In Fig. 2 there is shown a modication of the arrangement shown in Fig. 1 wherein the sharply tuned intermediate frequency amplifier includes thev tube 3 having its tuned input circuit` II inductively coupled to the resonant circuitrI, as at Mi. The sharp amplifier further includes an ampliiier 3 which has its input circuit I8 coupled to the tuned output circuit of tube 3, as at I3, the coupling I9 being less than critical coupling. AV tuned circuit 20 is magnetically coupled to the tunedoutput circuit of tube 3', the circuit 2B functioning as a phase correcting circuit.Y The sharp amplifier network ends with a circuit similar to that which begins the broad amplifier, and this makes the output phase angles of the two amplifiers similar.

The broadly tuned amplifier network includes the tube 6 which is coupled to the tuned circuit I through the condenser '1, the control grid of tube 6 being connected to the-gain control device I'I" through lead'l 1. The cathode of tube 6 is grounded. The amplifier tube I I succeeds the tube 3, and the output from circuit VII isimpressed'upon a resonant circuit 2|. The leads I6 and I1 are connected between the grid circuits of tubes Sand 6 respectively as in the case of Fig. l. The coupling M3, and the coupling between circuits VII and 2i is greater than critical coupling. A special tube 22 combines the outputs of the sharply tunedfamplier and the broadly tuned amplifier, the outputs of these two ampliiiers being impressed upon the grids 23 and 23 of tube 22, these two control grids being disposed in parallel relation.

The tube 22 includes a positive screen grid between the parallel grids 23, 23 and the anode of Y the tube.

The anode circuit of tube 22 is coupled to the rectifier input circuit IV'through a tuned circuit III. It is tol be clearly understood that the anode circuit of tube 22 can also be connected to an additional intermediate frequency ampliiier stage. The action of this circuit of Fig. 2 is the same as in Fig. 1.

In Fig. 3 is shown another circuit arrangement for parallel sharp and broad ampliiiers. In this circuit arrangement the sharp and broad amplifiers have the same type of input circuit. No special circuits are required to correct the phase angles Vfor this reason. Also, a different output combining circuit is shown utilizing two conventional screen grid Vtubes 3 and 6 with their anodes tied together by leads 4I! and 4I. v

"'I'he common 'tunedcircuit'42 has its highpotencuit 42 is magnetically coupled, asiat 43, to the The 'control biasesare applied to the sharp and broad amplifiers through leads I6 and I1 `respeclead I1 is connectedvto the grids of tubes 6, 6

and 6 from the control device I1'. VThecontrol biases are applied to the parallel amplifier systems in such a manner `that these systems are I' controlled differentially in accordance with the signal amplitude' variations as described herel toforei Y i Still another modification of the present invention is shown in Fig. 4. In this modification there is connected between the tuned input circuit IV of the detector 4 and the source of signal energy I a pair of cascaded tuned push-pull amplifier networks. The first of the cascaded stages comprises a pair of screen grid tubes 50 and I connected in push-pull relation, the signal control grids being connected through condensers to opposite sides of the tuned input circuit 52, the midpoint of the inductor circuit 52 being connected to ground through a radio frequency condenser 53. Y

The tuned circuits 54 and 55 are connected in series between the anode of tube 5G and the anode of tube 5I. Through a bias resistor 51 is furnished a fixed bias for the grid of tube 50, while through a resistor 51 is furnished control bias for the grid of tube 5I VThe second push-pull stage comprises a pair of screen grid tubes 60 and GI.

Y The ungrounded side of resistor I4 is connected tol the control grid circuit of tube 60 through a lead 52, a lead 63 and grid resistor 64, while the same point is connected to the control grid of tube 5I through a path which includes the lead 62, the lead 65 and the grid resistor 51.

A grid resisto-r 66 is connected between the cathode and grid of tube 6 I, a fixed source of grid bias voltage being included therein. When tubes 55, 5I, 60 and 6I are operating with equal control grid biases, the circuits 54 and 55, which are tuned to the carrier frequency, are effectively in parallel. In this condition the rresonance curve is the same as that of the ordinary form of amplifier employing one tuned circuit per stage. Now, if the biases on tubes 5I and 60 are increased so as to cut tubes 5I and 60 out of service, circuits 54 and 55 become a pair of coupled band passcircuits, tube 5B feeding into the primary and tube 6I taking the signal from the secondary. Ii the coupling M has the right value a band pass characteristic is obtained in` this stage. Of course, tubes 5B and 65 can be used instead of tubes 5u and 6I, but this will give less selectivity. It is desirable tof hold the centerpoint of each grid circuit of the cascaded pushpull amplifier tubes at ground potential. The coils and 1I are not tuned to signal frequency.

It will now be realized that there is shown in Fig. 4 an arrangement wherein a push-pull tuned amplifier, which is selective, is converted into a single tube coupled circuit system if certain tubes are made inoperative. When the signal amplitude increases the ungrounded side of resistor I4 becomes more negative, with the result that the biases applied to the grid circuits of tubes 65) and 5I become sufficiently great to render these two tubes inoperative.

As explained above this results in a transformationfof `the"cascaded push-pull vnetworks intoa band pass amplifier network wherein the tube 50 couplesthe resonant circuit 52 to the coupled tuned circuits 54 and 55, the latter coupled tuned circuits having the band pass characteristic, and the tube 6I deriving energy from the coupled circuits 54, 55 through the secondary :coil 1i), 1I. Thus, with strong signals the Vcircuit Ysystems for carrying my invention into effect, it will be apparent' to one skilled -in the art that said detectors, for differentially regulating the.

gain of said amplifiers. Y

2. In combination with a pair of amplifier circuits, one of which has a sharp resonance characteristic, and the other has a broad resonance characteristic, a rectifier coupled to each of said amplifiers, and means, responsive to variations in the output currents of said rectifiers, for differentially regulating the transmission efciency of said ampliers, and a tube coupling the rectier input circuits to the output circuits of said amplifiers, said tube including at least two control grids, one of which control grids is connected to one of the amplifiers, andthe other grid is connected to the remaining amplifier.

3. In a system as defined in claim 2, said control grids being arranged in parallel with each other, and a phase correcting circuit coupled to the output circuit of one of said amplifiers.

4. In an automatic fidelity control system for a radio receiver, a sharply tuned amplifier and a broadly tuned amplifier arranged in parallel, each of said ampliiiers including an output tube, a resonant circuit, means for connecting the output electrodes of said output tubes to the high potential side of said resonant circuit, a rectifier coupledv to said resonant circuit, and a conductive connection between gain control electrodes of said parallel amplifiers and gain control means connected to the rectifier for differentially regulating the gain of said ampliers.

5. In combination with a source of signalling energy and a rectifier adapted to convert impressed signal energy into direct current potential variations, an amplifier system comprising a pair of cascaded push-pull amplifier networks, said amplifier system including means for resonating the system to a desired carrier frequency, said system being constructed to have a sharply selective high frequency response characteristic when receiving weak signals, and a conductive connect-ion to at least one tube of each of the cascaded networks from the rectifier circuit for impressing on said tubes said direct current potential variation whereby said amplifier system is given a broad high frequency response characamplifier system coupled between said Vsourceand demodulator, said system comprising at Vvleast; a pair of cascaded signal amplifier networks arranged in parallel, each of the networks including at least one resonant circuit tuned to the operating signal frequency and arranged to couple successive tubes of the networks, said networks being constructed and arrangedto impart a sharp selectivity characteristic to the system when receiving weak signals and a relatively broad vselectivity characteristic when receiving strong signals, and additional means responsive to variations in received signal amplitude for regulating the gain of predetermined tubes of'said parallel networks to render one or the other ofrsaid characteristics predominant.

iaosrasoz 7. `In a receiver as dened in 'claim 6, thezsaid resonant Vcircuits of said parallel networks being -reactively coupled.

8. In combinationfwith` a sharply selective -signal amplifier and a broadly'tuned signal amplier, a common signal source coupled'to both ampliers, asignal detector coupled to theoutputs of said amplifiers,l means for regulating the gain of `sense as said signal strength variation.

9. In a system las dened in claim 8, said `regulating means comprising signal rectiers.

RENE A. BRADEN. 

